Radio Astronomers Set New Standard for Accurate
Cosmic Distance Measurement

A team of radio astronomers has used the National Science Foundation's
Very Long Baseline Array (VLBA) to make the most accurate measurement
ever made of the distance to a faraway galaxy. Their direct measurement
calls into question the precision of distance determinations made by other
techniques, including those announced last week by a team using the
Hubble Space Telescope.

The radio astronomers measured a distance of 23.5 million
light-years to a galaxy called NGC 4258 in Ursa Major. "Ours is a
direct measurement, using geometry, and is independent of all other
methods of determining cosmic distances," said Jim Herrnstein, of
the National Radio Astronomy Observatory (NRAO) in Socorro, NM.
The team says their measurement is accurate to within less
than a million light-years, or four percent. The galaxy is also
known as Messier 106 and is visible with amateur telescopes.

Herrnstein, along with James Moran and Lincoln Greenhill of the Harvard-
Smithsonian Center for Astrophysics; Phillip Diamond, of the Merlin radio
telescope facility at Jodrell Bank and the University of Manchester
in England; Makato Inoue and Naomasa Nakai of Japan's
Nobeyama Radio Observatory; Mikato Miyoshi of Japan's National Astronomical
Observatory; Christian Henkel of Germany's Max Planck Institute for Radio
Astronomy; and Adam Riess of the University of California at Berkeley,
announced their findings at the American Astronomical Society's
meeting in Chicago.

"This is an incredible achievement to measure the distance to another
galaxy with this precision," said Miller Goss, NRAO's Director of
VLA/VLBA Operations. "This is the first time such a great distance
has been measured this accurately. It took painstaking work on the
part of the observing team, and it took a radio telescope the size
of the Earth -- the VLBA -- to make it possible," Goss said.

"Astronomers have sought to determine the Hubble Constant, the rate of
expansion of the universe, for decades. This will in turn lead to an
estimate of the age of the universe. In order to do this, you need an
unambiguous, absolute distance to another galaxy. We are pleased that
the NSF's VLBA has for the first time determined such a distance, and
thus provided the calibration standard astronomers have always sought in
their quest for accurate distances beyond the Milky Way," said Morris
Aizenman, Executive Officer of the National Science Foundation's
(NSF) Division of Astronomical Sciences.

"For astronomers, this measurement is the golden meter stick in
the glass case," Aizenman added.

The international team of astronomers used the VLBA to measure
directly the motion of gas orbiting what is generally agreed to be a
supermassive black hole at the heart of NGC 4258. The orbiting
gas forms a warped disk, nearly two light-years in diameter, surrounding
the black hole. The gas in the disk includes water vapor, which,
in parts of the disk, acts as a natural amplifier of microwave
radio emission. The regions that amplify radio emission are called
masers, and work in a manner similar to the way a laser amplifies
light emission.

Determining the distance to NGC 4258 required measuring motions
of extremely small shifts in position of these masers as they
rotate around the black hole. This is equivalent to measuring an
angle one ten-thousandth the width of a
human hair held at arm's length. "The VLBA is the only instrument in
the world that could do this," said Moran.

"This work is the culmination of a 20-year effort at the Harvard
Smithsonian Center for Astrophysics to measure distances to cosmic
masers," said Irwin Shapiro, Director of that institution.

Collection of the data for the NGC 4258 project was begun in 1994
and was part of Herrnstein's Ph.D dissertation at Harvard
University.

Previous observations with the VLBA allowed the scientists to measure
the speed at which the gas is orbiting the black hole, some 39 million
times more massive than the Sun. They did this by observing the amount
of change in the wavelength of the radio waves caused by the Doppler
effect. The gas is orbiting at a speed of more than two million miles
per hour.

The orbiting disk of gas is almost edge-on as viewed from Earth.
The astronomers obtained the orbital speeds and the positions of the
masers in the disk by measuring the Doppler Shift of the masers at the
disk's sides, where the gas is moving almost directly away from the
Earth on one side and toward the Earth on the other. Measurements of
the different orbital speeds at different distances from the black hole,
made in 1994, allowed them to determine the mass of the black hole.
These measurements required the great resolving power, or ability to
see fine detail, of the VLBA.

This picture of an orbiting disk was confirmed by measurement of
centrifugal acceleration, according to the scientists.

The newest observations were focused on maser "spots" on the near edge
of the disk, where orbital motion shifts their position in the sky,
though by an extremely small amount. The VLBA,
however, was able to detect this extremely small movement, called
"proper motion" by astronomers. This motion was detected by
observing the galaxy at 4- to 8-month intervals over more than
three years.

"By knowing the speed at which the gas is orbiting and then
measuring its motion across the sky, we can use plain old
trigonometry to calculate the distance," Greenhill said. He
added, however, that "you need a bit of luck to be able to
do this. So far, we know of only 22 galaxies with water masers
in their nuclear regions that also are relatively nearby. Then,
the geometry of the disk, relative to Earth, has to be right
to allow us to make such a measurement"

The VLBA measurement of NGC 4258's distance differs significantly
from the distance to that galaxy determined through HST observations
of Cepheid variable stars. Using such stars, a team of astronomers
led by University of California-Berkeley scientist Eyal Maoz has made
preliminary and as-yet unpublished estimates of the distance to NGC 4258
as either 27 or 29 million light-years, depending on assumptions about
the characteristics of this type of star in that galaxy. Other Cepheid-based
galaxy distances were used to calculate the expansion rate of the
universe, called the Hubble Constant, announced by a team of HST
observers last week.

"This difference could mean that there may be more uncertainty in
Cepheid-determined distances than people have realized," said Moran.
"Providing this directly-determined distance to one galaxy -- a distance
that can serve as a milestone -- should be helpful in determining
distances to other galaxies, and thus the Hubble Constant and the size
and age of the universe"

The VLBA is a system of ten radio-telescope antennas, each 25 meters
(82 feet) in diameter, stretching some 5,000 miles from Mauna Kea
in Hawaii to St. Croix in the U.S. Virgin Islands. Operated from
NRAO's Array Operations Center in Socorro, NM, the VLBA offers
astronomers the greatest resolving power of any telescope anywhere.
The NRAO is a facility of the National Science Foundation,
operated under cooperative agreement by Associated Universities, Inc.

Background information: Determining Cosmic Distances

Determining cosmic distances obviously is vital to understanding the
size of the universe. In turn, knowing the size of the universe is
an important step in determining its age. "The size puts a limit on
how much expansion could have occurred since the Big Bang, and thus
tells us something about the age," said Moran.

However, determining cosmic distances has proven to be a particularly
thorny problem for astronomers. In the third century, B.C., the
Greek astronomer Aristarchus devised a method of using trigonometry
to determine the relative distances of the Moon and Sun, but in
practice his method was difficult to use. Though a great first
step, he missed the mark by a factor of 20. It wasn't until 1761
that trigonometric methods produced a relatively accurate distance to
Venus, thus calibrating the size of the Solar System. The first
accurate distance to another star was determined trigonometrically
by Friedrich Wilhelm Bessel in 1838.

Traditional trigonometric methods of measuring celestial distances
require extremely accurate measurement of an object's position in
the sky. By measuring the apparent shift in an object's position, called
parallax, caused by the Earth's journey around the Sun, the distance to
the object can be calculated. Until recent years, such measurements were
limited by the atmosphere's degrading effect on optical observations.
Recently, the Hipparcos satellite has measured stellar distances
accurate to within 10 percent out to about 300 light-years.

Beyond the range of parallax measurements, astronomers were forced
to use indirect methods of estimating distances. Many of these methods
make presumptions about the intrinsic brightness of objects, then
estimate the distance by measuring how much fainter they appear on
Earth. The faintness is presumed to be caused by the distance,
according to the inverse-square law (doubling of the distance
reduces brightness by a factor of four). Thus, stars of a particular
spectral class are all presumed to be of the same intrinsic
brightness. Such techniques have been used to estimate distances
of stars out to about 25,000 light-years, still not far enough
to estimate distance beyond our own Milky Way Galaxy.

Early in the 20th Century, Henrietta Leavitt, of Harvard College
Observatory, discovered that variable-brightness stars known as
Cepheid variables showed a useful property -- the longer
their pulsation periods, the brighter they are intrinsically. Once
the absolute distance to a few Cepheids was determined, these
stars were used to measure distances beyond the Milky Way.

In the 1920s, Edwin Hubble used Cepheid-variable distance
determinations to show that, contrary to then-prevalent opinion,
many "nebulae" were, in fact, other galaxies far distant from
our own. Distances determined using Cepheid variables, along
with measurements of the Doppler shift of other galaxies' light,
allowed Hubble to discover the expansion of the universe, the
basis of the Big Bang theory.

The Cepheid technique still is one of the building blocks of the
extragalactic distance scale. However, because of absorption of light by
interstellar dust and subtle differences among the stars themselves, this
technique is subject to considerable uncertainty.

Similarly, techniques that use a specific type of supernova (Type Ia)
presumed to be of uniform intrinsic brightness, while able to make
distance estimates farther than the Cepheid technique, still are subject
to uncertainties.

The NSF's VLBA, with resolving power hundreds of times better than even
the Hubble Space Telescope, has allowed direct trigonometric techniques
to be applied in measuring much greater distances than ever before.

The VLBA measured the expansion of the shell of exploding debris
from the supernova SN 1993J in the galaxy M81, 11 million light-
years away. This information, combined with optical observations
that measured the speed of the expanding debris by the Doppler shift
of its emitted spectral lines, allowed a trigonometric calculation
of the distance to M81.

Now, with the VLBA's direct measurement of motions in the gas disk
surrounding NGC 4258, trigonometric measurement, not subject to the
vagaries of dust absorption and other uncertaintities in an object's
brightness, has been extended to a distance of more than 23 million
light-years.